How to use the paint calculator
- Enter the Room length (ft), Room width (ft) and Ceiling height (ft). For a room that is not a plain rectangle, use the overall length and width.
- Type the Number of doors and Number of windows. Each door takes off 21 sq ft and each window 15 sq ft. Open Change door and window sizes if yours are bigger or smaller.
- Set Coats (two is normal) and Coverage (sq ft per gallon), which is printed on the can. The default of 350 is cautious, and 400 is common on smooth walls.
- Choose Paint the ceiling too? The ceiling is shown on its own line because it usually takes its own flat white paint.
- Read the gallons and the Buy line, which rounds up to the next quart.
How the gallons are worked out
Wall area is 2 x (length + width) x ceiling height. Subtract the doors and windows to get the paintable wall area, multiply by the number of coats, and divide by the coverage on the can. The ceiling is length x width, treated the same way. This Old House says a gallon of paint usually covers 350 to 400 square feet, and primer 200 to 300, in its paint estimating guide. That guide takes off 20 sq ft per door and 10 per window, so the 21 and 15 sq ft used here are slightly cautious and you can change them.
Worked example with the default inputs
A 15 x 12 ft room with 8 ft ceilings, one door, two windows, two coats and 350 sq ft per gallon. Wall area is 2 x (15 + 12) x 8 = 432 sq ft. The door and windows take off 21 + 2 x 15 = 51 sq ft, which leaves 381 sq ft per coat, or 762 sq ft over two coats. Divided by 350, that is 2.18 gallons, so the Buy line says 2 gallons and 1 quart. Switch on the ceiling and the 180 sq ft ceiling adds a second line of 1.03 gallons, or 1 gallon and 1 quart.
Limits and edge cases
- Trim, baseboards, crown molding, closets and built-ins are not included. Measure them separately and add them to your order.
- Bare drywall, stains, textured walls and a big color change use more paint than the can suggests. Plan a coat of primer, and lower Coverage if you are painting rough surfaces.
- For a vaulted room or one with a gable wall, enter the average wall height. For a stairwell, split it into parts and run each part.
- Planning other finishes in the same room? Try the wallpaper calculator for papered walls or the tile calculator for a backsplash. The area converter turns square meters into square feet.
Frequently asked questions
How much paint do I need for a 12 x 12 room?
With 8 ft ceilings, one door and one window, the walls are 384 sq ft minus 36 sq ft of openings, which is 348 sq ft per coat. Two coats is 696 sq ft, or 1.99 gallons at 350 sq ft per gallon, so buy 2 gallons.
How many square feet does a gallon of paint cover?
Most gallons cover 350 to 400 square feet with one coat on a smooth, previously painted wall. Rough, porous or bare surfaces soak up more, so read the coverage printed on your can and type it into the Coverage field.
Do I subtract doors and windows when figuring paint?
Yes, for the closest estimate. A standard door is about 21 sq ft and a typical window about 15 sq ft. If you would rather have extra for touch-ups, set both counts to 0 and the result will cover the full wall.
Should I buy all the paint at once?
Yes. Tinted paint can vary a little from batch to batch, so buy the full amount shown in the Buy line together, and keep any leftover for touch-ups.